In the art, techniques are known which allow employing combined PET-MRT systems to perform attenuation correction of acquired PET data based on acquired MRT data.
As part of the attenuation correction, the attenuation of PET photons, which are emitted due to an interaction of positrons and electrons, is determined for the path of the photons through absorbing tissue towards a PET detector of a PET imaging unit. The signal detected by the PET imaging unit by way of a PET detector is corrected to eliminate the determined attenuation of the PET data, i.e., PET counts (attenuation correction). Typically, the attenuation correction relies on an attenuation map (μ-map) which provides a linear attenuation coefficient (μ) or absorption value of the PET photons in a spatially resolved manner. The μ-map can be obtained from special MRT imaging sequences and post-processing techniques, such as a Dixon imaging sequence known to the skilled person. For this, MRT data is acquired which relates to the anatomy of the patient along the path of the PET photons. Therefore, MRT data is typically acquired for a region of interest to be imaged using the PET technique and additionally for a surrounding area along the path of the PET photons, i.e., which the PET photons have to cross in order to reach the PET detector of the PET imaging unit.
Generally, the accuracy of the attenuation correction of the acquired PET data may directly correlate with the signal-to-noise ratio or confidence level of a PET image which is reconstructed from the PET data. To this respect, physical values, which can be derived from the PET image or PET data, may be determined at a higher accuracy, i.e., with lower uncertainty, if attenuation correction is applied to the PET data. This may allow to achieve the technical effect of increased confidence of physical values determined from the PET data.
To this respect, in particular the breathing motion of the patient or physiological effects like cardiac motion can cause discrepancies between the PET data and the MRT data which degrade the attenuation correction. Such discrepancies may relate to a spatial mismatch and/or changes in the anatomy (lung volume, etc.). This can cause image artefacts and quantification errors which can, e.g. locally, affect the PET image quality and increase uncertainty in the derived physical values. This is in particular important for advanced and recent PET imaging techniques, like “motion freeze by gating”, etc.
Various techniques are known to address such problems and will be discussed in the following.
Some techniques, such as combined PET-computer tomography (CT) techniques allow acquiring, firstly, gated PET data, and secondly and subsequently, gated CT data for the attenuation correction. In particular, the acquisition of the PET data and the CT data may occur in two independent and sequential processes separated in time. Such techniques, however, may suffer from certain disadvantages and drawbacks due to the subsequent acquisition of the CT data, either prior to or after the acquisition of the PET data. For example, such a sequential approach is problematic if the breathing pattern of the patient differs between the PET data acquisition imaging period and the CT data acquisition imaging period.
Still further techniques employ software solutions which allow a retrospective and manual matching of the PET data with the CT or MRT data by medical personnel in order to provide accurate attenuation correction. However, such techniques may be time-consuming and error-prone.
Last, techniques are known in which the MRT data for the attenuation correction is acquired in an end expiration breath hold after the acquisition of the respective PET data. The position of the diaphragm of the patient's lung is close to an average position during the free breathing at the time of PET data acquisition. In some cases, there can be differences between the anatomical position of the breathhold and the free-breathing average position. This may cause motion artefacts.
MRT is an imaging technique which allows the acquisition of two-dimensional (2D) or three-dimensional (3D) MRT images based on acquired MRT data. MRT images can picture structures and objects in the interior of a patient with comparably high spatial resolution. In MRT techniques, magnetic nuclear spins in a region of interest are aligned using a static (DC) magnetic field such that a net macroscopic magnetisation is achieved. The magnetisation is subsequently exited out of its rest or equilibrium position (typically parallel to the DC magnetic field) using radio frequency (RF) pulses. The decay of the thus excited magnetisation back to the rest position, i.e., the magnetisation dynamics of the nuclear spins, is subsequently detected using RF-detector coils. A spatial encoding/selectivity of the acquired MRT data is achieved by applying gradient fields (for slice selection, phase selection, and frequency selection). The acquired and spatially resolved MRT data are present in wave vector space (k-space) and can be transformed into image space using (inverse) Fourier transformation. By applying the gradient fields in a particular manner, it is possible to sample k-space using various trajectories, i.e., follow a certain path through the k-space during the MRT data acquisition. Such techniques are known to the skilled person, such that there is no need to discuss further details in this context.
However, such techniques employing MRT data for the attenuation correction, suffer from certain drawbacks. For example, discrepancies between the anatomical positions of the region of interest of the patient may occur between the PET and MRT data, respectively. It may not or only to a lesser degree be possible to provide matched MRT data with no or only little motion difference to the respective PET data.